A light metal which is susceptible to oxidation such as aluminum or magnesium or an alloy thereof, or a transition metal such as iron or nickel or an alloy thereof is generally subjected to a surface treatment such as an anodizing treatment or a chemical conversion treatment for improving the corrosion resistance thereof.
The anodizing treatment is an anti-rust treatment in which a target metal is used as an anode in an electrolytic solution and a current is passed therethrough, whereby an oxide coating film is formed on the surface thereof.
The chemical conversion treatment is an anti-rust treatment in which a thin sulfide or oxide coating film is formed on the surface of a target metal in a solution by utilizing a chemical reaction such as sulfuration or oxidation. Examples thereof include a phosphate coating treatment which is a priming treatment for coating iron or steel, a chromate treatment which is a post-treatment of galvanization, and an iron blackening treatment.
On the other hand, in these treatments, a chromate, the use of which is largely restricted due to its harmfulness to the human body, and a fluoride, for which a wastewater treatment is difficult to increase the cost, are used, and so on, and therefore, a surface treatment method having a lower environmental load has been demanded.
PTL 1 discloses a steel plate subjected to a chromium-free treatment to impart heat resistance, in which a zinc alloy-plated steel plate substrate is coated with a coating film containing a liquid glass and at least one compound selected from at least a thiocarbonyl group-containing compound and a vanadate compound.